Creedmoor Sports Rimfire Rim Thickness Gauge The Creedmoor Sports Rimfire Rim Thickness Gauge is designed to safely measure the rim thickness of .22 Long Rifle cartridges. Sorting based on rim thickness can help increase the consistency of your ammunition which may lead to tighter groups. For use with Calipers: The top of the tool is where the brass thumbscrew is located. Loosen the thumb screw and slide the tool on the top jaw of the caliper. Close the calipers to the bottom jaw. Tighten the thumb screw. Zero the calipers. Place the .22 Long Rifle cartridge in the tool. Gently close the calipers and record the measurement. Anvil is not included. For use with the Comparator Stand: Remove the probe from the dial indicator. Place the screw through the center of the tool and thread into the dial indicator. Adjust the height of the dial indicator so the end of the tool rests squarely on the granite base. (It is usually best practice to "preload" the dial indicator by lowering the dial indicator so the needle travels 1 full revolution.) Lift the dial indicator stem, slide the cartridge under the tool, and gently lower the dial indicator. Record your measurements. Caution: In a rimfire cartridge, the priming compound is contained in the rim. Crushing the rim may cause the round to ignite. Our .22 Rimfire Rim Thickness Gauge has been designed not to contain pressure if the rim is inadvertently crushed. Do not drop the caliper or stem on the dial indicator while using this tool.